The present invention relates to a process for the production of an oriented monocrystalline silicon film with localized defects on an insulating support.
It more particularly applies to the field of producing MOS or bipolar integrated circuits, particularly with a g high operating speed, which resist ionizing radiations and/or opening under high voltage and being able to dissipate high power levels. It also applies to the production of the control circuits of matrix flat screens on transparent amorphous substrates, such as glass.
Silicon-on-insulant technology constitutes a significant improvement compared with standard technologies in which the active components of the integrated circuits are directly produced on a monocrystalline solid silicon substrate. Thus, the use of an insulating material leads to a significant decrease in the parasitic capacitances between the source and substrate on the one hand and the drain and substrate on the other of the active components of the integrated circuits and consequently to an increase in the operating speed of said circuits.
Moreover, this technology makes it possible to significantly decrease the parasitic charges introduced into the integrated circuits operating under ionizing radiation, in view of the fact that such radiations cannot ionize the insulating support on encountering a solid silicon support. Moreover, this technology makes it possible to increase the integration density of the integrated circuits, because they make it possible to avoid latch-up due to the breakdown of the junctions of said integrated circuits. Finally, this technology leads to a significant simplification of the processes for producing integrated circuits, as well as to a better resistance of said circuits to high voltages.
The obtaining of monocrystalline semiconductor films and in particular silicon films on insulating supports has been studied with a view to producing flat screens and there control circuits on transparent amorphous substrates. When the first experiments using laser beams or electron beams revealed that it was possible to grow silicon grans by heating them and particularly by melting the deposited silicon, as a result of a zone micromelting, increased activity took place with a view to obtaining monocrystalline silicon films on an insulating support.
This lead to a search for methods making it possible to rapidly treat industrial silicon wafers, as well as the search for heating devices with a lower cost than a laser source or an electron gun. Thus, use was made of graphite resistors or filament or arc lamps. These heating devices have more particularly been described in an article by M. W. GEIS et al, which was published in Applied Physics Letters, 37, May 1980, p454 and in French patent application No. 2 532 783 of 7.9.1982.
Unfortunately, none of these known heating methods has made it possible to produce monocrystalline silicon films without defects over the entire surface of a silicon wafer with industrial dimensions (diameter 100 mm).
All the experiments would seem to show that, even with non-coherent energy beams permitting the recrystallization of entire silicon wafers in a single passage, it is difficult to envisage eliminating the residual defects, such as grain boundaries and subboundaries or other dislocations. This is even more true in view of the fact that silicon films are thin, i.e. having a thickness below 0.5 .mu.m.
Moreover, in order to orient the recrystallized semiconductor film, attempts where made to use a fine grating etched in the insulating support in the manner described by M. M. GEIS et al, in Appl Phys Lett, H 35, July 1979, pp 71-74 "crystallographic orientation of silicon on an amorphous substrate using an artificial surface - relief grating and laser crystallization". Unfortunately the methods used in silicon zone melting, like those described in the article by M. W. GEIS and an FR-A-2 532 783 did not make it possible to obtain completely oriented, defect-free, recrystallized silicon wafers.
One of the most frequently used methods for obtaining a monocrystalline silicon film on an insulating support consists of forming on a monocrystalline silicon substrate of given orientation, a silicon oxide film, particularly by thermal oxidation and then depositing on the said film a polycrystalline silicon film and finally the film undergoes heat treatment, such as a scanning of a melted zone of the film in order to recrystallize the silicon.
In such a procedure, consideration was given to making openings in the silicon oxide film so as to use the monocrystalline substrate as a recrystallization germ or nucleus in order to orient the recrystallized film and prevent its disorientation, which are the main reasons why grain boundaries and subboundaries appear. This nucleation process is particularly described in the article by M. FEN published in Appl Phys Lett, 38, 1981, p 365.
In this procedure, it appeared necessary to reproduce with a short period (less than 500 .mu.m) the so-called nucleation openings in the silicon oxide film so as not to loose the initial orientation. However, this leads to the disadvantage of requiring, following the recrystallization of the silicon film, a localized oxidation stage of said nucleation zones in order to insulate the recrystallized semiconductor film from the monocrystalline substrate and to the be able to use the different, aforementioned advantages as compared with the silicon on-insulant technology. Moreover, there oxidized zones lead to a space loss, because it is not possible to produce integrated circuits insulated from one another on these oxidized zones.
Moreover, an investigation has taken place of the formation an appearance of grain boundaries and subboundaries with a view to discovering localization procedures for these defects in narrow zones where the active zones (the transistor gates) of the integrated circuits are not produced. In particular, by recrystallizing the semiconductor film on the insulating support by scanning a melted silicon zone, it was found that the boundaries and subboundaries of the grains formed in the zones which solidified last after the passage of the melted zone.
By using in this recrystallization method a laser source, where the energy can be concentrated on a very narrow spectrum, it is possible to couple to a greater or lesser extent the energy of the laser beam with the semiconductor film and to heat to a greater or lesser extent a particular zone of said film as compared with another zone. This can be realized, in the manner described in French patent application No. 83 16396 by using antireflecting parallel strips on the silicon film to be crystallized and which act as crystallization delaying means, and by calculating the thickness of the antireflecting strips and the reflecting strips (silicon strips), in order to adjust the energy of the laser beam interacting with the silicon film.
However, in the case of an incoherent light source, in view of the broad emission spectrum thereof, it is difficult to adjust the thickness of the reflecting and antireflecting strips in order to attempt to bring into phase the light waves striking the structure. However, as described in the article M. W. GEIS et al, published in J Electrochem Soc: Solid-state Science and Technology 130, pp 1178-1183 of May 1983 and entitled "Solidification-Front Modulation to Entrain Subboundaries in Zone Melting Recrystallization of Si on SiO.sub.2 ", use was made of black bodies or metal layers for their relecting properties in a broad spectrum.
In the same way, in an article by D. BENSAHEL, et al published in Electronics Letters of 23.6.1983, vol 19, No. 13, pp 464-466 and entitled "Localization of defects on SOI films via selective recrystallization using halogen lamps", use was made of deposited layers of different index as rough refractive index adapting means without using limited calibrated thicknesses.
In another process for the localization of residual defects in a recrystallized silicon film, the isotherms of the semiconductor melted zone where modified using a selective diffusion of the heat in the underlying substrate and varying the thickness of the silicon oxide film supporting the semiconductor film. This process using a laser source as the heating source was described in an article by S. KAMAMURA et al, published in J Appl Phys, 55 (6) of 15.3.1984, pp 1607 to 1609 entitled "Laser recrystallization of Si over SiO.sub.2 with a heat sink structure".
When it is possible to localize the different recrystallization defects of the silicon film, as described hereinbefore, the main remaining problem is that of the crystalline orientation of the recrystallized film. If with non-coherent energy beams the recrystallized film has a texture (100) perpendicular to the plane of the film, in the case of laser beams this film is not generally oriented and it is necessary to use germination openings towards the monocrystalline substrate made in the silicon oxide film. However, in the case of non-coherent energy beams, the silicon film remains disoriented in the plane of the film. The disorientation is approximately 30.degree. compared with the displacement direction of the melted zone.
In the case of non-coherent energy beams, other problems remain and in particular the behavior of the semiconductor films during zone melting. Thus, when the silicon is liquid and in contact with the silicon oxide, which is generally used as the insulating support, the silicon does not wet the insulant and has the tendency to form drops. In addition, it is necessary to find an effective "encapsulant", i.e. an element enclosing the semiconductor film making it possible to maintain the latter in contact with the insulating support and consequently prevent silicon drop formation, so as to limit the melted silicon mass transfer.
At present, it is attempted to prevent drop formation by covering the silicon film with a thick silicon oxide or silicon nitride layer (2 .mu.m for a 0.5 .mu.m silicon film). However, even with such material thicknesses, the silicon sometimes undergoes droplet formation, so that completely unusable silicon wafers are obtained.